ROCKPORT – Police are trying to learn why a driver heading west on Route 90 Thursday night was in the eastbound lane.

The car, driven by William Green, 35, of Waldoboro, struck another vehicle head-on, killing its driver, Stuart Hanna, 44, of Friendship. A third vehicle also was involved in the collision, though its occupants escaped serious injury.

Hanna was declared dead at the scene, according to police.

Rockport Police Chief Mark Kelley said the accident occurred at about 9:40 p.m., just west of the entrance to Camden Hills Regional High School. Green’s 1985 Oldsmobile was traveling west, but was entirely in the eastbound lane, he said.

That portion of the highway is wide and has broad shoulders, the chief noted.

Green’s car struck the newer model Ford Explorer “practically head-on,” killing Hanna, Kelley said.

The third vehicle, traveling east behind Hanna’s vehicle, was a newer-model Toyota SUV, he said. Its driver, Katherine Merrill, 41, of Camden, tried to steer to avoid the crash, but her vehicle struck Green’s car.

Merrill and her passenger, Cecelia Salas, 47, of Camden, were not seriously injured, Kelley said.

Green suffered serious head, chest and abdomen injuries and was taken by helicopter to Eastern Maine Medical Center in Bangor. A spokeswoman at EMMC said Green was in critical condition Friday afternoon.

Kelley said Bangor police will take a sample of Green’s blood to test for drug or alcohol content, as is the case with all fatal crashes. The chief said there was no evidence in Green’s vehicle to suggest he had been drinking.

The chief did not know whether Green was wearing a seat belt. Air bags in Hanna’s and Merrill’s vehicles did deploy, he said.

Mark Coleman, a crash reconstruction expert with the Maine State Police, is assisting Rockport Officer Travis Ford with the investigation, Kelley said. Officers with the Knox County Sheriff’s Department and Camden police assisted with traffic control Thursday night, as Route 90 had to be closed for more than five hours, he said.